Various improvements have been proposed for a battery case for housing a power generating unit of a battery.
For example, Japanese Unexamined Utility Model Publication No. Sho 56-127672 proposes improving a bottomed prismatic battery case made of resin (hereinafter “resin case”) for lead storage batteries. Specifically, the resin case houses an electrode assembly formed by laminating a plurality of positive electrodes and negative electrodes with a separator interposed between the positive electrode and the negative electrode. In order to prevent part of electrode active material separated from one of the positive and negative electrodes and dropped on the bottom of the resin case, if any, from coming in contact with the other electrode, the bottom of the resin case is provided with a space for receiving the dropped material. The space is obtained by forming a recessed portion by providing a slope on the bottom of the resin case.
Japanese Laid-Open Patent Publication No. 2008-130429 proposes providing a step portion on the bottom of a bottomed cylindrical battery case made of metal for alkaline batteries, the step portion being provided between a base portion on which a hollow cylindrical positive electrode is placed, and a protruding portion (a positive electrode terminal) protruding from the base portion. This step portion has a flat surface which is perpendicular to the axis direction of the positive electrode and on which a separator is placed.
Japanese Laid-Open Patent Publication No. Hei 10-321198 proposes using a bottomed cylindrical battery case made of metal for alkaline batteries, the battery case having a thickness of the bottom of 0.130 to 0.195 mm and a thickness of the side of 0.070 to 0.195 mm, for the purpose of increasing the capacity and reducing the weight of the batteries.
However, in alkaline batteries, there is a risk in the production process thereof that, from the inner wall of the hollow cylindrical positive electrode disposed in the bottomed cylindrical battery case, part of the positive electrode is separated, and part of the separated positive electrode drops on a portion of the bottom of the battery case on which a separator is to be placed. If the dropped material remains on the portion on which a separator is to be placed, this may cause a trouble such as failure of proper insertion of the separator. Such trouble is peculiar to alkaline batteries having an inside-out structure.
In alkaline batteries disclosed in Japanese Laid-Open Patent Publication No. 2008-130429 also, the above trouble may occur as a result that the dropped material remains on the step portion of the battery case. Even when the battery case disclosed in Japanese Laid-Open Patent Publication No. Hei 10-321198 is used in alkaline batteries, there is a risk that the above trouble occurs.